J. Electromagnetic Analysis & Applications, 2010, 2, 649-653
doi:10.4236/jemaa.2010.212085 Published Online December 2010 (http://www.SciRP.org/journal/jemaa)
Copyright © 2010 SciRes. JEMAA
A High-Luminescence Fringelike Field Emission
from Screen-Printed Carbon Nanotube and Zinc
Oxide Composite Film
Shiguang Shang1, Ling Zhao1, Weihua Liu2, Ping Zhao1, Xin Li2
1School of Electronic Engineering, Xi’an University of Posts and Telecommunications, Xi’an, China; 2School of Electronics and
Information Engineering, Xi’an Jiao Tong University, Xi’an, China.
E-mail: shangshiguang05@163.com
Received October 27th, 2010; revised November 20th, 2010; accepted November 28th, 2010.
A fringelike field emission with high-luminescence and stable emission current from screen-printed carbon nanotube
mixed zinc oxide (CNT-ZnO) composite cathode was investigated. The luminescent patterns are significantly different
from those observed in the field emission measure of pure CNT cathode. SEM images reveal that the CNTs are perfectly
matched with ZnO powders by filling the interspaces in CNT film. XRD analysis demonstrates that the CNTs and ZnO
have a high degree of crystalline perfection. Field emission measurement exhibits that the turn-on field of CNT-ZnO
cathode is 2.08 V/µm, lower than 2.46 V/µm for pure CNT cathode. The large fringelike emission current at th e brims
of CNT-ZnO catho de is attributed to a combination of the in creased effective contact area of CNTs, which decrease the
sheet resistance of cathode film, and the dangled CNT bundles at th e brims of CNT-ZnO film cathode.
Keywords: Carbon Nanotube, Fringelike Field Emission, Film Cathode
1. Introduction
Since the discovery of carbon nanotubes (CNTs) [1], the
application of CNTs as field emitters has been widely
investigated for the possible development of electronic
and photonic nanodevices, such as field emission dis-
plays, X-ray sources and microwave devices [2-4].
Chemical vapor deposition (CVD) and screen-printing
are two widely used methods for CNT film fabrication
[5-8]. Screen-printed carbon nanotube film has attracted
much attention in field emission display owing to its po-
tentials of low production costs, simple manufacturing
process and uniformity in large area [9,10]. However, the
CNTs in screen-printed thin film cathode are randomly
distributed, and their emission capabilities are poor
compared with vertically aligned CNTs in high ly ordered
arrays [8,11,12]. Therefore, special treatments are needed
to improve the field emission performances of screen-
printed CNT films. Recently, various treatment tech-
niques, such as ion irradiation, plasma procedure and
laser treatment, have been employed [13-15]. Observed
changes in surface morphologies suggest that these meth-
ods will break up th e C N T bundles into individual free-
standing CNTs by damaging the original structure of
CNTs. However, Lou et al. [16] studied the method us-
ing powder metallurgy to enhance the adhesion between
CNTs and substrate, and thus improve CNT field emis-
sion characteristics. The result indicated that the cathodes
fabricated by this method have typical I-V curve and
stable field emission property. In the paper, CNTs were
mixed with ZnO powders in order to improve the field
emission properties of screen-printed CNT film cathodes,
and the influence of ZnO powder on the field emission
property of CNT thin film is analyzed.
2. Experiment Details
Multi-walled carbon nanotubes (MWCNTs) prepared by
catalytic chemical vapor deposition (Shenzhen Nano Port)
were used as emitters and ZnO powders were used as
bonding materials in the screen-printed cathode films.
There are three main steps in the CNT-ZnO cathode
films. Firstly, the raw CNTs were purified with a mixture
of concentrated nitric acid and concentrated sulphuric
acid to remove amorphous carbon and catalyst particles,
followed by deioned water washing, filtering and drying
under vacuum at room temperature. Secondly, the puri-
A High-Luminescence Fringelike Field Emission from Screen-Printed Carbon Nanotube and Zinc Oxide Composite Film
fied CNTs were dispersed into terpineol by ultrasonica-
tion for a long time, followed by filtration to remove the
large particles. Then ethyl cellulose was mixed with the
solution by stirring and heating it up to120°C until ethyl
cellulose was fully dissolved. Then we got pure CNTs
paste for screen-printing. Following the same procedure,
we got the ZnO powder paste. Finally, the two pastes
were mixed (50 wt%: 50 wt %) and screen-printed onto
silver coated glass substrates. After being lift in clean
room for 24 hours, the CNT-ZnO film cathodes were
sintered at 596°C for 60 minutes in argon to remove the
organic binder.
The surface morphologies of the pure CNT and
CNT-ZnO films were characterized by field emission
scanning electron microscopy (FESEM, JSM-6700F,
Japan). The structural information of CNT-ZnO film was
analyzed by small angle X-ray diffraction in Shimadzu
diffractometer (XRD-6000, Japan) using Cu Ka radiation.
The field emission characteristics were tested with a pla-
nar diode configuration. The distance between cathode
and anode was fixed at 200 μm and the emission area
was 1 2 cm2. The measurements were carried out at
room temperature in a vacuum chamber at 10-6 Pa base
3. Results and Discussion
Figures 1(a-c) show the SEM images of pure CNT and
CNT-ZnO screen-printed film cathodes after sintering.
As shown in Figure 1(a), the pure CNT film shows a
homogeneous and clean surface, where the CNTs were
loose and randomly oriented. Figure 1(b) shows the
morphology of the CNT-ZnO film cathode. The CNTs
and ZnO powder formed a continuous conductive layer
and the film show some roughness with island of
CNT-ZnO agglomerations. On the assumption that the
protruded tips and/or arcs of CNTs were potential emit-
ters, the emitter density of CNT-ZnO film was decreased
and only the protuberances of CNTs were formed the
field emission tips. However, the roots of CNTs were
embedded into ZnO agglomeration, which would give a
significant improvement of the electrical contact of
CNTs to the substrate. Figure 1(c) shows the cross-sec-
tion view of the CNT-ZnO film cathode. The pure CNTs
film is a layer of loose network on the substrate. While,
in the CNTs-ZnO composite film, ZnO grains geometri-
cally matched with CNTs by filling into the interspaces
of CNTs or directly covering upon CNTs, thus few pro-
truding CNT emitters on the surface of CNT-ZnO com-
posite film were obtained. Nevertheless, many dangle
bundles are observed on the cross-section of CNT-ZnO
film cathode, which can deduce that there are many po-
tential emitters at both of the brims of CNT-ZnO com-
posite film.
Figure 1. Scanning electron microscopy images of screen-
printing CNT cathode films: (a) Pure CNT film, (b) CNT-
ZnO film and (c) Cross-section of CNT-ZnO film.
Copyright © 2010 SciRes. J
A High-Luminescence Fringelike Field Emission from Screen-Printed Carbon Nanotube and Zinc Oxide Composite Film 651
Figure 2 shows the XRD patterns obtained from
theCNT-ZnO composite film. To avoid the appearance of
silver diffraction peaks, the CNT-ZnO paste was printed
onto glass substrates directly. As shown in the XRD pat-
tern, the curve is pertinent to the diffraction peaks both
from hexagonal wurtzite structure zinc oxide and graph-
ite structure CNTs. The dominant peaks of hexagonal
wurtzite structure zinc oxide are at 2θ = 31.7, 34.3, 36.2
and 56.5, corresponding to (100), (002), (101) and (110)
crystal orientations, respectively. Three representative
peaks of graphite structure at 2θ = 26.0, 42.6 and 54.7,
corresponding to (022) (100) and (004) crystal
orientations, are also observed. The XRD peaks of CNTs
are narrower and sharper than that of the untreated sam-
ples [17], indicating that the crystallinity of the CNTs
was improved. The main reasons are contributed to re-
move of carbonaceous particles, such as fullerenes,
nanoparticles and amorphous phases at high sintering.
The high crystallinity is beneficial for field emission
Figures 3(a,b) show the lighting images of pure CNT
and CNT-ZnO film cathode. Figure 3(a) is the typical
field emission pattern of the pure CNT film at applied
voltage of 1.26 kV, in which the emission sites were
sporadic and nonuniform. Figure 3(b) is the field emis-
sion pattern of the CNT-ZnO film at applied voltage of
1.12 kV. As shown in Figure 3(b), the emission sites
were located at both brims of the CNT-ZnO films, and
the luminescence areas stretches outwards from the brims
of CNT-ZnO film cathode. The luminescence areas show
a fringelike shape, and which are similar to the contrails
of pendulum in simple harmonic motion. During the field
emission of the CNT-ZnO films, the luminescence areas
would change from a uniform and static pattern to an
unstable flash one with the increas ing of applie d voltage.
Figure 2. X-ray diffraction patterns of CNT-ZnO composite
Figure 3. Field emission images of screen-printing cathode
films: (a) Pure CNT film, (b) CNT-ZnO composite film.
The main reason was supposed to be the electron-wave
interferences. The electron-wave interference may be
generated from the coherent transport properties of the
neighbor CNT emission sites in CNT-ZnO dangled bun-
dles, which were in good agreement with the emission
pattern from Pt field emitter fabricated by electron-
beam-induced deposition [19]. Generally, the vibration
amplitude corresponding to low applied voltage is too
small to detect. Further work is needed to explore the
fundamental physics.
For the CNT-ZnO composite film cathode, we believe
that the fringelike field emission is closely related with
several facts. Firstly, the interspaces of CNTs were filled
with ZnO grains and the CNT-ZnO film was formed a
good conductiv e layer. The conductiv e bonding materials
would result in low sheet resistance of screen-printed
films so that the penetration of the electrical field can be
ignored, leading to th e low field enhancement factor [20].
Copyright © 2010 SciRes. JEMAA
A High-Luminescence Fringelike Field Emission from Screen-Printed Carbon Nanotube and Zinc Oxide Composite Film
Secondly, some CNT-ZnO agglomerations island, as
shown in Figures 1(b,c), are higher than surface CNT
emitters on CNT-ZnO film cathode, and thus suppressed
the field enhancement effects of emission sites. Thirdly,
the CNTs at the brims of the CNT-ZnO film cathode are
easier to form dangled bundles, and the screening effect
on the CNTs near the brims is lower than that on the sur-
face CNTs of CNT-ZnO film. In addition, low contact
resistance can be formed from silver electrode to CNT
emitters. Therefore, the CNT emitters at the brims of
CNT-ZnO film cathode would have good emission prop-
erty and high saturation current density. As a result, the
fringelike luminescence pattern was formed.
Figure 4 shows the field emission current densities of
pure CNT and CNT-ZnO films as a function of applied
voltage. The turn-on electric fields (defined as the elec-
tric field when the emission current density reaches 10
µA/cm2) of the pure CNT and the CNT-ZnO films are
2.46 V/ µm and 2.08 V/ µm, respectively. Compared to
the pure CNT film cathode, the CNT-ZnO film cathode
has lower turn-on electric field and higher field emission
current density. The results indicate that the mixing of
CNT and ZnO powder can enhance the field emission
ability of CNTs. It is mainly contributed to the formation
of high contact area and low contact resistance between
CNTs and substrate. Therefore, the barrier on the junc-
tion of CNTs to substrate would be eliminated, and elec-
trons can pass through this jun ction without ob stacle [21].
During the whole field emission, electrons only need to
overcome CNT-vacuum barrier, a low voltage would
result in a considerable electron emission. The emission
degradation behavior of emission current density was
also measured. After 200 minutes operation with initial
emission current density of 200 µA/ cm2, current degen-
eration of CNT-ZnO film cathode less than 5% were ob-
served. The results indicated that the field emission of
CNT-ZnO film cathode was stable.
Figure 4. Field emission I-V curves of different screen-
printing CNT films: (a) pure CNT film, (b) CNT-ZnO com-
posite film.
4. Conclusions
A composite film cathode of CNT-ZnO was fabricated to
improve the field emission performances of CNT emit-
ters, and a high-luminescence fringelike field emission
was observed. The experiment results indicate the
CNT-ZnO film cathode has three characteristics as fol-
lowing. Firstly, most of the emission sites showing high
intensity emission locate at both brims of the CNT-ZnO
composite film. And small part of emission sites locate at
the center of cathode film, which has lower field emis-
sion capability than that of pure CNTs. Secondly, field
emission electrons from neighboring emitters at the
brims of CNT-ZnO cathode film maybe generate elec-
tron-wave interferences, and which would cause dynamic
field emission pattern. Finally, the fringelike field emis-
sion of CNT-ZnO film cathode exhibits lower turn-on
field, higher luminescence intensity and excellent emis-
sion current stability. In a summary, the CNT-ZnO film
provides a promising cold cathode for lighting sources,
such as field emission lighting tube and back light.
5. Acknowledgements
The work was financially supported from the grant from
the 863-Programme of the Ministry of Science and
Technology of China (No. 2008AA03A314), and Xi’an
Applied Materials Innovation Fund Application No.
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